1. Field of the Invention
The present invention relates to a video tape recorder (VCR) of the helical scanning rotary head type for recording and reproducing video signals.
2. Description of Prior Art
A known component type VCR for recording and reproduction of video signals employs a method in which a timebase reference for the video signal is produced by inserting several sinewaves of color burst (referred to as a burst wave) after a negative synchronizing (sync) pulse for one horizontal scanning period, prior to frequency modulation of the video signal for recording.
In reproduction of the video signal with this method, a zero-cross point of the burst wave in a demodulated video signal is detected for producing a playback pulse following a timebase variation. The playback pulse then actuates a PLL circuit to produce a playback clock signal.
The demodulated video signal containing a timebase variation is converted to a digital signal by an A/D converter controlled by the playback clock and stored in a memory. It is then read out from the memory by a reference clock which carries no timebase variation so that a resultant reproduced video signal is free from timebase variation.
However, the burst wave which is used for timebase correction is inserted in each horizontal scanning period and the timebase frequency of the NTSC system becomes as low as about 15 KHz. If the video signal is timebase extended for each horizontal scanning period and is recorded on a plurality of channels, the timebase frequency will further be decreased to about 7.5 KHz.
When the timebase correction is carried out at such a low frequency, its response speed will remain low. Accordingly, an abrupt timebase change caused by the switching of heads affects video data recorded in the beginning of each recording track. This develops a duration, equal to a multiple of H (H: horizontal scanning period), where no timebase variation can be corrected throughout several horizontal scanning periods. As the result, a visual failure known as skew distortion will appear in the upper portion of a reproduced image.
For eliminating the above failure, a modified method has been proposed as disclosed in laid-open Japanese Patent Application No. S63-61577, in which a negative signal which is longer in duration than a negative sync pulse provided in each horizontal scanning period is inserted in the beginning of each track prior to recording, and this negative signal is detected for timebase correction during reproduction.
However, even if the duration of the negative signal is adequately long, only one edge is used as a timebase reference. This provides a precision of timing almost equal to that given by the negative sync pulse in each horizontal scanning period and the effect of skew distortion will hardly be eliminated.